Many designs of radiotherapy apparatus employ a radiation source mounted on a gantry arm that is rotatable around a patient support on which a patient can be placed for treatment. The usual geometry for this is to provide a circular-section rotatable drum oriented in a vertical plane, i.e. with its axis of symmetry and rotation in a horizontal plane, and mount the gantry arm onto the drum, offset from the axis. The source is then mounted at the end of the gantry arm, oriented so that the beam that it produces is directed towards the axis. The point at which the centre of the beam meets the axis is known as the “isocentre”. Thus, as the drum rotates the beam arrives at the isocentre from all angular directions within a vertical plane. This is an important aspect of the radiotherapy treatment, as it allows a sufficient dose to be delivered to a target volume while minimising the dose delivered to surrounding healthy tissue.
Usually, the rotating drum is supported on four main wheels beneath the drum, arranged in two angularly-offset pairs, one pair at a front edge of the drum and one pair at a rear edge. The drum and the gantry arm are usually substantial items in order to support the weight of the radiation source mounted in the arm and at the end thereof. Despite this, there will be some small degree of flexure in the gantry arm, resulting in a “droop’ effect, i.e. an unintended movement of the isocentre. With the gantry at the top of the drum (defined as 0° of rotation) the movement of the isocentre Is along the axis towards the drum, whereas at 180° rotation with the gantry at the bottom of the drum, the movement of the isocentre is along the axis away from the drum. Whilst this is a known, measurable effect that can be planned and compensated for during treatment it would be additionally beneficial to minimise the effect where possible.